SiO2-layered double hydroxide microspheres and methods of making them

ABSTRACT

A method of making silica-layered double hydroxide microspheres having the formula I: (i) wherein, M z+  and M′ y+  are two different charged metal cations; z=1 or 2; y=3 or 4; 0&lt;x&lt;0.9; b is 0 to 10; c is 0 to 10; P&gt;0, q&gt;0, X n−  is an anion; with n&gt;0 a=z(1−x)+xy−2; and the AMO-solvent is an 100% aqueous miscible organic solvent; comprises the steps: (a) contacting silica microspheres and a metal ion containing solution containing metal ions M z+  and M′ y+  in the presence of a base and an anion solution; (b) collecting the product; and (c) optionally treating the product with AMO-solvent and recovering the solvent treated material to obtain the silica-layered double hydroxide microspheres. Preferably, M in the formula I is Li, Mg, Ni or Ca. Preferably, M′ in formula I is Al. The invention further provides silica-layered double hydroxide microspheres having the formula I. The silica-layered double hydroxide microspheres may be used as catalysts and/or catalyst supports.
 
(SiO 2 ) p @{[M z+   (1-x) M′ y+   x (OH) 2 ] a+ (X n− ) a/n   .b H 2 O. c (AMO-solvent)} q   (I)

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national stage entry under 35 USC § 371 of PCT International Application Number PCT/GB2016/050024, filed Jan. 6, 2016, which claims priority to United Kingdom Patent Application Number 1500115.9, filed Jan. 6, 2015, the entire disclosures of which are expressly incorporated by reference herein.

The present invention relates to new SiO₂-layered double hydroxide (LDH) microspheres and to methods of making the same.

Layered double hydroxides (LDHs) are a class of compounds which comprise two or more metal cations and have a layered structure. A review of LDHs is provided in Structure and Bonding; Vol. 119, 2005 Layered Double Hydroxides ed. X Duan and D. G. Evans. The hydrotalcites, perhaps the most well-known examples of LDHs, have been studied for many years. LDHs can intercalate anions between the layers of the structure.

Core shell particles are described in the literature by “core@shell” (for example by Teng et al., Nano Letters, 2003, 3, 261-264, or by “core/shell” (for example J. Am. Chem Soc., 2001, 123, pages 7961-7962). We have adopted the “core@shell” nomenclature as it is emerging as the more commonly accepted abbreviation.

SiO₂/LDH core-shell microspheres are described by Shao et al, Chem. Mater. 2012, 24, pages 1192-1197. Prior to treatment with a metal precursor solution, the SiO₂ microspheres are primed by dispersing them in an Al(OOH) primer sol for two hours with vigorous agitation followed by centrifuging, washing with ethanol and drying in air for 30 minutes. This priming treatment of the SiO₂ microspheres was repeated 10 times before the SiO₂ spheres thus coated with a thin Al(OOH) film were autoclaved at 100° C. for 48 hours in a solution of Ni(NO₃)₂.6H₂O and urea. Hollow SiO₂—NiAl-LDH microspheres obtained by this process were reported as exhibiting excellent pseudocapacitance performance. Unfortunately, the requirement for the Al(OOH) priming of the SiO₂ surface, prior to LDH growth, makes this process unsuitable for use on an industrial scale.

Chen et al, J. Mater. Chem. A, 1, 3877-3880 describes the synthesis of SiO₂@MgAl-LDHs having use in the removal of pharmaceutical pollutants from water. The synthesis described comprises coprecipitating LDH from a metal precursor solution containing the SiO₂ microspheres followed by ultrasound assisted direct growth of LDH nanosheets on the surface of the SiO₂ microspheres. Unfortunately, the reported method does not allow the morphology of the surface LDHs to be tuned and the surface area of the product SiO₂@LDHs is not high.

It is an object of the present invention to provide a facile method of making SiO₂@LDH microspheres which overcomes drawbacks of the prior art, which in particular does not require a step of initially forming an Al(OOH) film on the SiO₂ surface prior to treatment with the metal precursor solution or the requirement for ultrasound assistance in achieving LDH growth on the SiO₂ surface. It is also an object of the present invention to provide SiO₂@LDHs wherein the thickness, size and morphology of the LDH layer can each be tuned easily for different applications. It is yet a further object of the present invention to provide SiO₂@LDHs that have high surface area.

This object is achieved by a method of making silica-layered double hydroxide microspheres having the general formula I (SiO₂)_(p)@{[M^(z+) _((1-x))M′^(y+) _(x)(OH)₂]^(a+)(X^(n−))_(a/n) .bH₂O.c(AMO-solvent)}_(q)  (I) wherein, M^(z+) and M′^(y+) are two different charged metal cations; z=1 or 2; y=3 or 4; 0<x<0.9; b is 0 to 10; c is 0 to 10; p>0, q>0; X^(n−) is an anion; with n>0 (preferably 1-5) a=z(1−x)+xy−2; and the AMO-solvent is an 100% aqueous miscible organic solvent; which method comprises the steps:

-   (a) contacting silica microspheres and a metal ion containing     solution containing metal ions M^(z+) and M′^(y+) in the presence of     a base and anion solution; -   (b) collecting the product; and -   (c) optionally treating the product with AMO-solvent and recovering     the solvent treated material.

According to a further aspect of the invention, there is provided a silica-layered double hydroxide microsphere or a plurality thereof obtained, directly obtained or obtainable by a process defined herein.

A silica-layered double hydroxide microsphere is known to comprise a silica microsphere having solid LDH attached to its surface. Such a material, denoted as SiO₂@LDH, may be a core-shell material where the SiO₂ microsphere is a solid sphere, a yolk-shell material where the SiO₂ microsphere comprises an outer shell and a smaller SiO₂ sphere contained within the outer shell wherein there is a hollow portion between the smaller sphere and the inner surface of the outer shell, or a hollow shell material wherein the SiO₂ microsphere has a hollow interior.

In an embodiment, the process does not include a step of growing the LDH on the SiO₂ surface using ultrasound.

The SiO₂ microspheres used in the preparation of the SiO₂@LDH microspheres according to the invention may be solid, yolk-shell or hollow microspheres and are commercially-available in a variety of sizes (diameters). However, SiO₂ microspheres may be prepared by a modified Stöber process involving ammonia catalysed hydrolysis and condensation of tetraethylorthosilicate using ethanol as solvent and cetyltrimethylammonium bromide as surfactant porogen, as is well known in the art.

In an embodiment, the silica microspheres do not contain any iron.

In another embodiment, the silica microspheres comprise greater than 75% w/w SiO₂. Suitably, silica microspheres comprise greater than 85% w/w SiO₂. More suitably, the silica microspheres comprise greater than 95% w/w SiO₂. Most suitably, the silica microspheres comprise greater than 98% w/w SiO₂.

In another embodiment, the silica microspheres consist essentially of SiO₂.

In another embodiment, the silica microspheres consist of SiO₂.

In another embodiment, the SiO₂ microspheres have an average diameter of between 0.15 μm and 8 μm. Suitably, the SiO₂ microspheres have an average diameter of between 0.15 μm and 2 μm. More suitably, the SiO₂ microspheres have an average diameter of between 0.15 μm and 1 μm. Most suitably, the SiO₂ microspheres have an average diameter of between 0.2 μm and 0.8 μm.

The LDH nanosheets grown on the surface of the SiO₂ microspheres comprise, and preferably consist of, LDH represented by the general formula I [M^(z+) _(1-x)M′^(y+) _(x)(OH)₂]^(a+)(X^(n−))_(a/n) .bH₂O.c(AMO-solvent)  (I), wherein M^(z+) and M′^(y+) are different charged metal cations; z=1 or 2; y=3 or 4; 0<x<0.9; b=0-10; c=0-10, X^(n−) is an anion, n is the charge on the anion, n>0 (preferably 1-5); a=z(1−x)+xy−2; and AMO-solvent is an 100% aqueous miscible organic solvent.

As stated above, M^(z+) and M′^(y+) are different charged metal cations. Having regard to the fact that z=1 or 2, M will be either a monovalent metal cation or a divalent metal cation. When z=1, M is either a single monovalent metal cation or two or more different monovalent metal cations. When z=2, M is either a single divalent metal cation or two or more different divalent metal cations. In an embodiment, z=2, i.e. M is one or more divalent metal cations. M′, in view of the fact that y=3 or 4, will be a trivalent metal cation or a tetravalent metal cation. When z=3, M′ is either a single trivalent metal cation or two or more different trivalent metal cations. When z=4, M′ is either a single tetravalent metal cation or two or more different tetravalent metal cations. In an embodiment, y=3, i.e. M′ is one or more trivalent metal cations.

A preferred example of a monovalent metal, for M, is Li. Examples of divalent metals, for M, include Ca, Mg, Zn, Fe, Co, Cu and Ni and mixtures of two or more of these. Preferably, the divalent metal M, if present, is Ca, Ni or Mg. Examples of metals, for M′, include Al, Ga, In, Y and Fe. Preferably, M′ is a trivalent cation, for example Al. Preferably, the LDH will be a Li—Al, an Mg—Al or a Ca—Al LDH.

The anion X^(n−) in the LDH is any appropriate inorganic or organic anion. Examples of anions that may be used, as X^(n−), in the LDH include carbonate, hydroxide, nitrate, borate, sulphate, phosphate and halide (F⁻, Cl⁻, Br⁻, I⁻) anions. Preferably, the anion X^(n−), is selected from CO₃ ²⁻, NO₃ ⁻ and Cl⁻.

The AMO-solvent is any aqueous miscible organic solvent, i.e. a solvent which is 100% miscible with water. Examples of suitable water-miscible organic solvents for use in the present invention include one or more of acetone, acetonitrile, dimethylformamide, dimethylsulfoxide, dioxane, ethanol, methanol, n-propanol, isopropanol, or tetrahydrofuran. Suitably, the water-miscible organic solvents for use in the present invention is selected from lower (1-3C) alkanols and acetone. Preferably, the AMO-solvent is methanol, ethanol or acetone, especially acetone and ethanol.

According to one preferred embodiment, the layered double hydroxides are those having the general formula I above

in which M^(z+) is a divalent metal cation;

M′^(y+) is a trivalent metal cation; and

each of b and c is a number>zero, which gives compounds optionally hydrated with a stoichiometric amount or a non-stoichiometric amount of water and/or an aqueous-miscible organic solvent (AMO-solvent), such as acetone or ethanol.

Preferably, in the LDH of the above formula, M is Mg or Ca and M′ is Al. The counter anion X^(n−) is typically selected from CO₃ ²⁻, OH⁻, F⁻, Cl⁻, Br⁻, I⁻, SO₄ ²⁻, NO₃ ⁻ and PO₄ ³⁻. In a most preferred embodiment, the LDH will be one wherein M is Mg, M′ is Al and X^(n−) is CO₃ ²⁻.

In carrying out the method of the invention, preferably the SiO₂ microspheres are dispersed in an aqueous solution containing the desired anion salt, for example Na₂CO₃. A metal precursor solution, i.e. a solution combining the required monovalent or divalent metal cations and the required trivalent cations may then be added, preferably drop-wise, into the dispersion of the SiO₂ microspheres. Preferably, the addition of the metal precursor solution is carried out under stirring. The pH of the reaction solution is preferably controlled within the pH range 8 to 12, typically 8-11, more preferably 9 to 10.

In an embodiment, the method of the present invention does not include ammonia.

The structure of the SiO₂@LDH microspheres can be controlled by the tendency of SiO₂ to dissolve or partially dissolve at higher temperatures and higher pH values. For instance, carrying out the addition of the metal precursor solution to the dispersion of SiO₂ microspheres at room temperature and at pH 10 gives solid SiO₂@LDH particles although by raising the temperature of the reaction solutions, for instance to 40° C., it is possible to obtain yolk shell SiO₂@LDH particles. By carrying out the reaction at room temperature but using a higher solution pH, e.g. pH 11, it is possible to produce hollow shell SiO₂@LDH particles.

Typically, NaOH may be used to adjust the pH of the solution.

During the reaction, the LDH produced from the metal precursor solution reaction is formed on the SiO₂ surfaces of the microspheres as nanosheets.

It is preferred that the temperature of the metal ion containing solution in step (a) is within a range of from 20 to 150° C. More preferably, from 20 to 80° C.

In a preferred embodiment, the silica-layered double hydroxide microspheres have specific surface area of at least 100 m²/g, preferably at least 177 m²/g, and more preferably at least 200 m²/g, and even more preferably at least 250 m²/g.

Suitably, the silica-layered double hydroxide microspheres comprise solid silica mircrosphere cores and have specific surface area of at least 100 m²/g.

Suitably, the silica-layered double hydroxide microspheres comprise yolk-shell mircrosphere cores and have specific surface area of at least 100 m²/g.

Suitably, the silica-layered double hydroxide microspheres comprise hollow-shell mircrosphere cores and have specific surface area of at least 130 m²/g.

Most preferred, the silica-layered double hydroxide microspheres have at least one structure from hollow-shell, yolk-shell and solid core-shell structures.

In a preferred embodiment, the silica-layered double hydroxide microspheres have a thickness of layered double hydroxide layer larger than 65 nm, preferably larger than 110 nm, more preferably larger than 160 nm.

The obtained solid product is collected from the aqueous medium. Examples of methods of collecting the solid product include centrifugation and filtration. Typically, the collected solid may be re-dispersed in water and then collected again. Preferably, the collection and re-dispersion steps are repeated twice.

The finally-obtained solid material may then be subjected to drying, for instance, in an oven for several hours.

In the event that a product containing AMO-solvent is required, the material obtained after the collection/re-dispersion procedure described above may be washed with, and preferably also re-dispersed in, the desired solvent, for instance acetone or ethanol. If re-dispersion is employed, the dispersion is preferably stirred. Stirring for more than 2 hours in the solvent is preferable. The final product may then be collected from the solvent and then dried, typically in an oven for several hours.

In an embodiment, the LDH layer is formed in situ. Suitably, the LDH is formed and coated onto the silica microspheres in situ.

The growth of LDH nanosheets on the surface of the SiO₂ microspheres is “tuneable”. That is to say, by varying the chemistry of the precursor solution, pH of the reaction medium and the rate of addition of the precursor solution to the dispersion of SiO₂ microspheres, the extent of, and the length and/or thickness of, the LDH nanosheets formed on the SiO₂ surface can be varied.

The production of the SiO₂@LDH microspheres according to the invention can be carried out as a batch process or, with appropriate replenishment of reactants, as a continuous process.

The products of the method of the present invention where the SiO₂@LDHs are treated with solvent, i.e. the products having the formula I above where c is greater than zero, are a solution to the problem underlying the present invention per se.

Thus, according to a further aspect, the present invention provides silica-layered double hydroxide microspheres having the formula I (SiO₂)_(p)@{[M^(z+) _((1-x))M′^(y+) _(x)(OH)₂]^(a+)(X^(n−))_(a/n) .bH₂O.c(AMO-solvent)}_(q)  (I) wherein, M^(z+) and M′^(y+) are two different charged metal cations; z=1 or 2; y=3 or 4; 0<x<0.9; b is 0 to 10; c is 0 to 10; p>0, q>0; X^(n−) is an anion; with n>0 (preferably 1-5) a=z(1−x)+xy−2; and AMO-solvent is an 100% aqueous miscible organic solvent.

M′ in formula I above is preferably Al.

M in formula I above is preferably Li, Mg, Ga, In, Ni, Co, Cu or Ca, more preferably Mg or Ni.

According to an embodiment, X^(n−) in formula I above is selected from carbonate, nitrate, borate, sulphate, phosphate, hydroxide and halide anions (F⁻, Cl⁻, Br⁻, I⁻). Preferably, the anion X^(n−) is selected from CO₃ ²⁻, NO₃ ⁻ and Cl⁻.

According to a particularly preferred embodiment, the SiO₂@LDH microspheres of the invention contain LDH having the formula I above in which M is Mg, M′ is Al and X^(n−) is CO₃ ⁻.

The AMO-solvent in formula I above is one that is miscible in water, especially one that is 100% miscible in water. Examples of solvents that can be used include methanol, ethanol and acetone. According to a preferred embodiment of the invention, the solvent is ethanol or acetone.

SiO₂@LDHs according to the present invention may be used as catalysts and/or catalyst supports.

PARTICULARLY PREFERRED EMBODIMENTS

The following represent particular embodiments of the silica-layered double hydroxide:

-   1.1 The silica-layered double hydroxide microspheres have the     general formula I     (SiO₂)_(p)@{[M^(z+) _((1-x))M′^(y+) _(x)(OH)₂]^(a+)(X^(n−))_(a/n)     .bH₂O.c(AMO-solvent)}_(q)  (I)     -   wherein,     -   M^(z+) is selected from Li⁺, Ca²⁺, Ni²⁺ or Mg²⁺, and M′^(y+) is         Al³⁺ or Fe³⁺;     -   0<x<0.9;     -   b is 0 to 10;     -   c is 0 to 10;     -   p>0,     -   q>0;     -   X^(n−) is selected from carbonate, hydroxide, nitrate, borate,         sulphate, phosphate and halide (F⁻, Cl⁻, Br⁻, I⁻) anions; with         n>0 (preferably 1-5)     -   a=z(1−x)+xy−2; and     -   the AMO-solvent is selected from a lower (1-3C) alkanol (e.g.         ethanol) or acetone. -   1.2 The silica-layered double hydroxide microspheres have the     general formula I     (SiO₂)_(p)@{[M^(z+) _((1-x))M′^(y+) _(x)(OH)₂]^(a+)(X^(n−))_(a/n)     .bH₂O.c(AMO-solvent)}_(q)  (I)     -   wherein,     -   M^(z+) is selected from Li⁺, Ca²⁺, Ni²⁺ or Mg²⁺, and M′^(y+) is         Al³⁺;     -   0<x<0.9;     -   b is 0 to 10;     -   c is 0 to 10;     -   p>0,     -   q>0;     -   X^(n−) is selected from CO₃ ²⁻, NO₃ ⁻ or Cl⁻; with n>0         (preferably 1-5)     -   a=z(1−x)+xy-2; and     -   the AMO-solvent is acetone or ethanol. -   1.3 The silica-layered double hydroxide microspheres have the     general formula Ia     (SiO₂)_(p)@{[M^(z+) _((1-x))Al³⁺ _(x)(OH)₂]^(a+)(X^(n−))_(a/n)     .bH₂O.c(AMO-solvent)}_(q)  (Ia)     -   wherein,     -   M^(z+) is selected from Li⁺, Ca²⁺, Ni²⁺ or Mg²⁺;     -   0<x<0.9;     -   b is 0 to 10;     -   c is 0 to 10;     -   p>0,     -   q>0;     -   X^(n−) is selected from CO₃ ²⁻, NO₃ ⁻ or Cl⁻; with n>0         (preferably 1-5)     -   a=z(1−x)+xy−2; and     -   the AMO-solvent is acetone or ethanol. -   1.4 The silica-layered double hydroxide microspheres have the     general formula Ia     (SiO₂)_(p)@{[Mg²⁺ _((1-x))Al³⁺ _(x)(OH)₂]^(a+)(X^(n−))_(a/n)     .bH₂O.c(AMO-solvent)}_(q)  (Ia)     -   wherein,     -   0<x<0.9;     -   b is 0 to 10;     -   c is 0 to 10;     -   p>0,     -   q>0;     -   X^(n−) is selected from CO₃ ²⁻, NO₃ ⁻ or Cl⁻; with n>0         (preferably 1-5)     -   a=z(1−x)+xy-2; and     -   the AMO-solvent is acetone or ethanol. -   1.5 The silica-layered double hydroxide microspheres have the     general formula Ib     (SiO₂)_(p)@{[Mg²⁺ _((1-x))Al³⁺ _(x)(OH)₂]^(a+)(CO₃ ²)_(a/n)     .bH₂O.c(acetone)}_(q)  (Ib)     -   wherein,     -   0<x<0.9;     -   b is 0 to 10;     -   c is 0 to 10;     -   p>0,     -   q>0;     -   a=z(1−x)+xy−2. -   1.6 The silica-layered double hydroxide microspheres have the     general formula Ic     (SiO₂)_(0.04)@{[Mg_(0.75)Al_(0.25)(OH)₂](CO₃)_(0.125).1.34(H₂O)}_(0.05)  (Ic) -   1.7 The silica-layered double hydroxide microspheres have the     general formula Id     (SiO₂)_(0.04)@{[Mg_(0.75)Al_(0.25)(OH)₂](CO₃)_(0.125).0.29(H₂O).0.15(acetone)}_(0.05)  (Id)

Preferred, suitable, and optional features of any one particular aspect of the present invention are also preferred, suitable, and optional features of any other aspect.

FIGURES

FIG. 1. XRD patterns showing three sizes of silica nanoparticles (a) 300 nm, (b) 550 nm and (c) 800 nm.

FIG. 2. TGA curves of differently sized silica nanoparticles (a) 800 nm, (b) 550 nm and (c) 300 nm.

FIG. 3. SEM images of 800 nm silica microspheres prepared via seeded growth.

FIG. 4. XRD patterns of (a) 550 nm silica microspheres, (b) LDH nano-particles and (c) SiO₂@LDH microspheres ((b) and (c) were synthesized according to example 1).

FIG. 5. Percentage weight loss of (a) LDH, (b) SiO₂@LDH microspheres and (c) silica nanoparticles ((a) and (b) were synthesized according to example 1).

FIG. 6. ²⁹Si Solid-state NMR of (a) silica microspheres and (b) SiO₂@LDH microspheres ((b) were synthesized according to example 1).

FIG. 7. XRD patterns of SiO₂@LDH microspheres prepared at different pH conditions (a) ammonia method (example 4), (b) pH 9, (c) pH 10 and (d) pH 11 ((b)-(d): example 1).

FIG. 8. TGA of SiO₂@LDH microspheres prepared at different pH conditions (a) pH 11, (b) pH 10, (c) ammonia method and (d) pH 9.

FIG. 9. TEM images of SiO₂@LDH microspheres synthesized according to example 1 except at different temperatures (a) room temperature (b) 40° C.

FIG. 10. TGA and dTGA curves of SiO₂@LDH microspheres prepared according to example 1 except at different temperature (a) room temperature and (b) 40° C., (i) TGA curve, (ii) dTGA curve.

FIG. 11. Solid state NMR (a) ²⁹Si (b) ²⁷Al. SiO₂. @ LDH prepared according to example 1 except at different temperature (i) at room temperature (ii) at 40° C.

FIG. 12. XRD patterns of SiO₂@LDH microspheres prepared with different Mg:Al ratios (a) 1:1 (example 2), (b) 3:1 (example 3).

FIG. 13. XRD patterns of SiO₂@LDH microspheres prepared according to example 1 (a) conventional water washing (b) acetone washing.

FIG. 14. TGA of SiO₂@LDH microspheres prepared according to example 2 (a) conventional water washing (b) acetone washing.

FIG. 15. TEM image of SiO₂@LDH microspheres with different ratio of Mg/Al (a) 1:1 (example 2), (b) 2:1 (example 1) and (c) 3:1 (example 3).

FIG. 16. TEM image of SiO₂@LDH microspheres according to example 1 except with different size of silica (a) 300 nm, (b) 550 nm and (c) 800 nm.

FIG. 17. TEM image of SiO₂@LDH microspheres with different morphology (a) solid (example 1), (b) yolk-shell (example 1 at 40° C.) and (c) hollow (example 1 at pH 11).

FIG. 18. XRD patterns of SiO₂@AMO-LDH with an Mg:Al=3:1 (a) pH=10 and room temperature (b) pH=10 and 40° C. (c) pH=11 and 40° C.

FIG. 19. XRD patterns of SiO₂@AMO-LDH with Mg:Ni:Al=2.7:0.3:1 (a) pH=10 and room temperature (b) pH=10 and 40° C. (c) pH=11 and 40° C.

FIG. 20. XRD patterns of SiO₂@AMO-LDH with an Mg:Al:Fe=3:0.9:0.1 (a) pH=10 and room temperature (b) pH=10 and 40° C. (c) pH=11 and 40° C.;

FIG. 21. TEM image of SiO₂@AMO LDH microspheres according to examples 5 and 7 at pH=10 and room temperature (a) Mg:Al=3:1 (b) Mg:Al:Fe=3:0.9:0.1.

FIG. 22. TEM image of SiO₂@AMO-LDH with Mg:Ni:Al=2.7:0.3:1 microspheres with different morphology according to example 6 (a) pH=10 and room temperature (b) pH=10 and 40° C. (c) pH=11 and 40° C.

EXPERIMENTAL METHODS

1. General Details

1.1 Powder X-Ray Diffraction

Powder X-ray diffraction (XRD) data were collected on a PANAnalytical X'Pert Pro diffractometer in reflection mode and a PANAnalytical Empyrean Series 2 at 40 kV and 40 mA using Cu Kα radiation (α1=1.54057 Å, α2=1.54433 Å, weighted average=1.54178 Å). Scans were recorded from 5°≤0≤70° with varying scan speeds and slit sizes. Samples were mounted on stainless steel sample holders. The peaks at 43-44° are produced by the XRD sample holder and can be disregarded.

1.2 Thermogravimetric Analysis

Thermogravimetric analysis (TGA) measurements were collected using a Netzsch STA 409 PC instrument. The sample (10-20 mg) was heated in a corundum crucible between 30° C. and 800° C. at a heating rate of 5° C. min-1 under a flowing stream of nitrogen.

1.3 Solid State NMR Spectroscopy

²⁹Si and ²⁷Al MAS NMR spectra were recorded on a Varian Chemagnetics CMX Infinity 200 (4.7 T). Samples were packed in 7.5 mm zirconia rotors. A double resonance MAS probe was used for all measurements and a MAS rate of 4 kHz for ²⁹Si, whereas MAS rate of 6 kHz was used for ²⁷Al. ²⁷Al MAS NMR spectra were acquired with a single pulse excitation applied using a short pulse length (0.7 μs). Each spectrum resulted from 2000 scans separated by 1 s delay. The ²⁷Al chemical shifts are referenced to an aqueous solution of Al(NO₃)₃ (δ=0 ppm). In order to obtain the quantitative ²⁹Si DPMAS NMR spectra, 5000 transients were typically acquired with an acquisition time of 68 ms (1024 data points zero filled to 16K) and recycle delay of 30 s. All ²⁹Si spectra were externally referenced to kaolinite (taken to be at δ=−91.7 ppm on a scale where δ(TMS)=0 ppm) as a secondary reference.

1.4 Transmission Electron Microscopy

Transmission Electron Microscopy (TEM) analysis was performed on a JEOL 2100 microscope with an accelerating voltage of 200 kV. Particles were dispersed in water or ethanol with sonication and then cast onto copper grids coated with carbon film and left to dry.

1.5 Scanning Electron Microscopy

Scanning Electron Microscopy (SEM) analysis was performed on a JEOL JSM 6610 scanning electron microscope. Particles were dispersed in water and cast onto a clean silica wafer. Before imaging, the samples were coated with a thin platinum layer to prevent charging and to improve the image quality. Energy dispersive X-ray spectroscopy (EDX), also carried out on this instrument, was used to determine the relative quantities of constituent elements on the surface of the sample.

1.6 Brunauer-Emmett-Teller Surface Area Analysis

Brunauer-Emmett-Teller (BET) specific surface areas were measured from the N₂ adsorption and desorption isotherms at 77 K collected from a Quantachrome Autosorb surface area and pore size analyser.

Example 1

Silica spheres (100 mg, 550 nm) were dispersed in deionised water (20 mL) using ultrasound treatment. After 30 min., Na₂CO₃ (0.96 mmol) was added to the solution and a further 5 min of sonication was carried out to form solution A. Next an aqueous solution (19.2 mL) containing Mg(NO₃)₂.6H₂O (0.96 mmol) and Al(NO₃)₃.9H₂O (0.48 mmol) was added at a rate of 60 mL/h to solution A under vigorous stirring at room temperature. The pH of the reaction solution was controlled to be 10 with the addition of 1 M NaOH. The obtained solid was collected with centrifugation at 4000 rpm for 5 min and then re-dispersed in deionised water (40 mL) and stirred for 1 h. The collection and re-dispersion were repeated twice. Afterward, the solid was washed with acetone (40 mL) and then re-dispersed in acetone (40 mL) and left to stir overnight. The solid was then dried under vacuum.

The SiO₂@LDH obtained in this Example, before the treatment with acetone, has the formula:— (SiO₂)_(0.04)@{[Mg_(0.75)Al_(0.25)(OH)₂](CO₃)_(0.125).1.34(H₂O)}_(0.05) The SiO₂@AMO-LDH, obtained after acetone treatment, has the formula:— (SiO₂)_(0.04)@{[Mg_(0.75)Al_(0.25)(OH)₂](CO₃)_(0.125).0.29(H₂O).0.15(acetone)}_(0.05)

Yolk shell particles were obtained by carrying out the addition of the aqueous solution containing the Mg(NO₃)₂.6H₂O and Al(NO₃)₃.9H₂O at 40° C. and pH10.

Hollow shell particles were obtained by carrying out the addition of the aqueous solution containing Mg(NO₃)₂.6H₂O and Al(NO₃)₃.9H₂O at room temperature but at pH11.

Surface Area Analysis

The solid SiO₂@LDH, the yolk shell SiO₂@LDH and the hollow shell SiO₂@LDH prepared as described above but without acetone treatment were subjected to Brunauer-Emmett-Teller (BET) surface area analysis.

The N₂ BET surface areas of the products were:

BET surface area (m²q⁻¹) Solid (i.e. core-shell) SiO₂@LDH 107 Yolk-shell SiO₂@LDH 118 Hollow-shell SiO₂@LDH 177

The BET surface areas reported above may be favourably compared to those of SiO₂@LDHs prepared according to (A) Shao et al. Chem. Mater. 2012, 24, pages 1192-1197 and to those of SiO₂@LDHs prepared according to (B) Chen et al. J. Mater. Chem. A, 1, 3877-3880.

BET surface area (m²q⁻¹) (A) SiO₂ microspheres pre-treated with Al(OOH). Product SiO₂@NiAl LDH. Solid (i.e. core-shell) SiO₂ microspheres 42.3 Yolk-shell SiO₂@LDH microspheres 68 Hollow-shell SiO₂@LDH microspheres 124 (B) SiO₂ microspheres-no pre-treatment-ammonia method. Product SiO₂@LDH. Solid (i.e. core-shell) SiO₂@LDH microspheres 61

Core-shell SiO₂@LDHs were prepared according to the procedures described in Example 1 and in the Examples 2 and 3 below having different thicknesses of LDH layer. The ratio of Mg/Al was varied to control the thickness of the LDH layer. A Mg:Al ratio of 1:1 was found to give an LDH layer of thickness 65 nm, a ratio of 2:1 was found to give an LDH layer of thickness 110 nm and a layer of thickness of 160 nm was obtained using a Mg:Al ratio of 3:1. TEM images are shown in FIG. 15. Core-shell SiO₂@LDHs were also prepared according to the procedure described in Example 1 above using different sized SiO₂ microspheres, 300 nm, 550 nm and 800 nm. TEM images are shown in FIG. 16. TEM images of the SiO₂@LDHs produced with different morphology (a) solid (Example 1), (b) yolk shell (Example 1 at 40° C.) and (c) hollow (Example 1 at pH11), as described above, are shown in FIG. 17.

Example 2

In order to obtain a 1:1 Mg:Al LDH, the procedure described above in Example 1 was repeated with the exception that an aqueous solution (19.2 mL) containing Mg(NO₃)₂.6H₂O (0.72 mmol) and Al(NO₃)₃.9H₂O (0.72 mmol) was added at a rate of 60 mL/h to solution A under vigorous stirring.

Example 3

In order to obtain a 3:1 Mg:Al LDH, the procedure described above in Example 1 was repeated with the exception that an aqueous solution (19.2 mL) containing Mg(NO₃)₂.6H₂O (1.08 mmol) and Al(NO₃)₃.9H₂O (0.36 mmol) was added at a rate of 60 mL/h to solution A under vigorous stirring. The XRD patterns of the SiO₂@LDH samples prepared with Mg:Al ratios of 1:1 (Example 2) and 3:1 (Example 3) are shown in FIG. 12.

Example 4

The silica@LDH particles were synthesised via the coprecipation method. Silica spheres (100 mg, 550 nm) were dispersed in deionised water (20 mL) using ultrasound treatment. After 30 min, the anion salt (0.96 mmol), Na₂CO₃, was added to the solution containing ammonia (0.8 mL, 35%) and a further 5 min of sonication was carried out to form solution A. Next an aqueous solution (19.2 mL) containing Mg(NO₃)₂.6H₂O) (0.96 mmol) and Al(NO₃)₃.9H₂O (0.48 mmol) was added at a rate of 60 mL/h to solution A under vigorous stirring. The obtained solid was collected with centrifugation at 4000 rpm for 5 min and then re-dispersed in deionised water (40 mL) and stirred for 1 h. The collection and re-dispersion were repeated twice. Afterward, the solid was washed with acetone (40 mL) and then re-dispersed in acetone (40 mL) and left to stir overnight. The solid was then dried under vacuum. The suspension was then dried under vacuum for materials characterisation.

The features disclosed in the foregoing description, in the claims as well as in the accompanying drawings, may both separately and in any combination thereof be material for realizing the invention in diverse forms thereof.

Example 5

In order to obtain Silica@AMO-LDHs in Mg:Al=3:1. Synthesise the Silica@LDH particles by using the co-precipitation method, disperse silica spheres (100 mg) in the deionised water (20 mL) by using ultrasound treatment for 30 min, add the anion salt Na₂CO₃ (0.96 mmol) in the solution and further treat by ultrasound for 5 min, the finally solution named A. Then add an aqueous solution (19.2 mL) containing (1.08 mmol) Mg²⁺ and (0.36 mmol) Al³⁺ in the solution A at the rate of 60 mL/h with vigorous stirring. The pH of the reaction solution is controlled with the addition of 1 M NaOH by an autotitrator. And the morphology of Silica@LDH is controlled by pH and temperature. The obtained solid is collected with centrifugation at 5000 rpm for 5 min and then re-dispersed in deionised water (40 mL) and stir for 1 h, the washing need repeated twice. Before final isolation, the solid is washed with acetone (40 mL) and left to stir over night, and the suspension is then dried under vacuum

Example 6

In order to obtain Silica@AMO-LDHs in Mg:Ni:Al=2.7:0.3:1. The Silica@LDH particles will be synthesized by using the co-precipitation method, disperse silica spheres (100 mg) in the deionised water (20 mL) by using ultrasound treatment for 30 min, add the anion salt Na₂CO₃ (0.96 mmol) in the solution and further treat by ultrasound for 5 min, the finally solution named A. Then add an aqueous solution (19.2 mL) containing (0.972 mmol) Mg²⁺, (0.108 mmol) Ni²⁺ and (0.36 mmol) Al³⁺ in the solution A at the rate of 60 mL/h with vigorous stirring. The pH of the reaction solution is controlled with the addition of 1 M NaOH by an autotitrator. As followed the morphology of Silica@LDH is controlled by pH and temperature. The obtained solid is collected with centrifugation at 5000 rpm for 5 min and then re-dispersed in deionised water (40 mL) and stir for 1 h, the washing need repeated twice. Before final isolation, the solid is washed with acetone (40 mL) and left to stir over night, and the suspension is then dried under vacuum.

Example 7

In order to obtain Silica@AMO-LDHs in Mg:Al:Fe=3:0.9:0.1. The Silica@LDH particles synthesise using the co-precipitation method, disperse silica spheres (100 mg) in the deionised water (20 mL) by using ultrasound treatment for 30 min, add the anion salt Na₂CO₃ (0.96 mmol) in the solution and further treat by ultrasound for 5 min, the finally solution named A. Then add an aqueous solution (19.2 mL) containing (1.08 mmol) Mg²⁺, (0.324 mmol) Al³⁺ and (0.036 mmol) Fe³⁺ in the solution A at the rate of 60 mL/h with vigorous stirring. The pH of the reaction solution is controlled with the addition of 1 M NaOH by an autotitrator. As followed the morphology of Silica@LDH is controlled by pH and temperature. The obtained solid is collected with centrifugation at 5000 rpm for 5 min and then re-dispersed in deionised water (40 mL) and stir for 1 h, the washing need repeated twice. Before final isolation, the solid is washed with acetone (40 mL) and left to stir over night, and the suspension is then dried under vacuum 

The invention claimed is:
 1. A method of making silica-layered double hydroxide microspheres, each having a silica microsphere core and a layered double hydroxide coating on the surface of the silica microsphere core, the silica-layered double hydroxide microspheres having the formula I (SiO₂)_(p)@{[M^(z+) _((1-x))M ^(y+) _(x)(OH)₂]^(a+)(X^(n−))_(a/n) .bH₂O.c(AMO-solvent)}_(q)  (I) wherein, M^(z+) and M′^(y+) are two different charged metal cations; z=1 or 2; y=3 or 4; 0<x<0.9; b is 0 to 10; c is 0 to 10; p>0, q>0, X^(n−) is an anion; with n>0 a=z(1−x)+xy−2; and the AMO-solvent is an 100% aqueous miscible organic solvent; which method comprises the steps: (a) contacting: (i) silica microspheres, (ii) a metal ion containing solution containing metal ions M^(z+) and M′^(y+), and (iii) an anion solution in the presence of a base to grow a layered double hydroxide coating on the surface of the silica microspheres; (b) collecting the product; and (c) optionally treating the product with AMO-solvent and recovering the solvent treated material to obtain the silica-layered double hydroxide microspheres; wherein the pH of the reaction solution formed in step (a) is controlled within a range from 8 to 12; and wherein the silica-layered double hydroxide microspheres have a thickness of layered double hydroxide of larger than 110 nm.
 2. The method according to claim 1, wherein M′ is one or more trivalent metal cations.
 3. The method according to claim 1, wherein M is one or more divalent metal cations.
 4. The method according to claim 1, wherein X^(n−) is selected from CO₃ ²⁻, OH⁻, F⁻, Cl⁻, Br⁻, I⁻, SO₄ ²⁻, NO₃ ⁻ and PO₄ ³⁻, or a mixture thereof.
 5. The method according to claim 1, wherein M is Mg, M′ is Al and X^(n−) is CO₃ ²⁻.
 6. The method according to claim 1, wherein the pH is controlled within a range from 9 to
 10. 7. The method according to claim 1, wherein the product collected in step (b) is dispersed in the AMO-solvent and wherein c in the formula I has a value which is greater than zero and up to
 10. 8. The method according to claim 7, wherein the dispersion of the product in the AMO-solvent is maintained for 2 or more hours and then the treated material is separated from the solvent.
 9. The method according to claim 1, wherein the AMO-solvent is acetone, ethanol or methanol or a mixture thereof.
 10. The method according to claim 1, wherein the temperature of the metal ion containing solution in step (a) is within a range of from 20 to 150° C.
 11. The method according to claim 10, wherein the temperature is from 20 to 80° C.
 12. The method of claim 1, wherein the silica-layered double hydroxide microspheres have specific surface area as determined by N₂ BET of at least 177 m²/g.
 13. The method of claim 1, wherein the silica-layered double hydroxide microspheres have at least one structure from hollow-shell, yolk-shell and solid core-shell structures.
 14. The method of claim 1, wherein step (a) comprises: dispersing the silica microspheres in an aqueous solution containing a salt of the anion, thereby forming a dispersion; and adding the metal ion containing solution containing metal ions M^(z+) and M′^(y+) to the dispersion, with stirring. 